In recent years, owing to the demands for higher integration degrees and higher operation speeds of LSIs, design rules for semiconductor devices that constitute LSIs are increasingly miniaturized. Along with this trend, the resistivity of transistor gate electrodes used in DRAMs, flash memories, and so forth is required to be lower. Conventionally, poly-silicon is used for gate electrodes, but entailing a disadvantage such that it increases the sheet resistance. Accordingly, it has been proposed to laminate a poly-silicon layer with a refractory metal, such as tungsten, or silicide of the metal, which is low in resistivity, high in adhesion to a silicon oxide film or silicon itself, and good in processability. Specifically, a tungsten polycide gate (a laminated film of WSi/poly-Si) and a tungsten poly-metal gate (a laminated structure of W/WN/poly-Si) with a lower resistivity have attracted attention. The WN of the tungsten poly-metal gate serves as a barrier layer (diffusion prevention layer) for preventing a reaction between tungsten and poly-silicon.
In general, where a transistor gate is set up, a well (impurity diffusion layer), a gate insulating film, and a gate electrode are formed in this order. When the gate electrode is formed, an etching process is performed. With this process, a side surface of a poly-silicon layer included in the gate electrode is exposed. When a voltage is applied to the gate electrode, an electric field is concentrated at this exposed portion, which may cause malfunctions of the product, such as an increase in leakage current. Accordingly, it is necessary to perform a selective oxidation process (so-called re-oxidation process) to oxidize the exposed poly-silicon portion of the gate electrode, thereby forming an insulating film thereon.
As a re-oxidation method for forming an insulating film on the exposed portion of a poly-silicon layer on the side surface of a gate electrode, it is typical to adopt a method using a thermal oxidation process at a high temperature of 800° C. or more. However, tungsten and tungsten silicide that are used for decreasing the sheet resistance of gate electrodes are rapidly oxidized when the temperature exceeds about 300° C. and about 400° C., respectively. Consequently, when a thermal oxidation process is performed on a gate electrode, a tungsten layer is also oxidized and WO3 is thereby generated. The WO3 thus generated brings about cubical expansion that narrows the inside of holes, as well as an increase in resistivity. Thus, a problem arises such that the resistance of the gate electrode is increased. Further, WO3 may be scattered and contaminate a wafer, thereby causing a problem in that the process yield for semiconductor devices is decreased. Furthermore, at such a high temperature, tungsten may react with poly-silicon and increase the resistivity due to diffusion of tungsten nitride (WN) of the diffusion prevention layer. In addition, thermal oxidation processes entail difficulty in forming an optimum sidewall oxide film of high quality, and need a relatively long time. Consequently, thermal oxidation processes make it difficult not only to control formation of the oxide film but also to improve the throughput, thereby decreasing the productive efficiency.
On the other hand, as a method for forming an oxide film other than a thermal oxidation process, there has been proposed a method by use of plasma for forming an oxide film on a substrate for, e.g., liquid crystal displays, although this method is not for the purpose of re-oxidation of a gate electrode (for example, Patent Document 1). According to this method, a silicon oxide film is deposited on a substrate under the action of plasma to form the silicon oxide film. At this time, hydrogen gas is supplied in addition to a silicon-containing gas and an oxygen-containing gas into a process chamber to generate hydrogen-containing plasma. With this arrangement, it is expected to attain a film of high quality comparable to thermal oxide films.
In the method of Patent Document 1 described above, the hydrogen plasma is used for filling defects in the oxide film, which suits for formation of an oxide film of high quality on a substrate. However, this Document suggests no application to selective re-oxidation for a poly-silicon layer included in a gate electrode.
According to studies made by the present inventors, it has been confirmed that, where the poly-silicon layer of a gate electrode having a metal layer, such as a tungsten layer, is preferentially oxidized by plasma, if hydrogen plasma is present, oxidation of the metal layer is suppressed to a considerable extent. However, it has been found that, even if oxidation of the tungsten layer is suppressed, a large amount of tungsten is mixed into semiconductor devices as a metal contaminant at atomic level, and the contamination thus caused reaches a level not negligible for the performance of the semiconductor devices. In other word, tungsten thus mixed may affect semiconductor devices, such as transistors, and cause malfunctions of the product, thereby decreasing the process yield.
Further, the present inventors have tried to perform a re-oxidation process by use of a plasma processing apparatus of the RLSA (Radial Line Slot Antenna) microwave plasma type, which can realize a low temperature process by plasma having a high plasma density and a low electron temperature.
However, it has been confirmed that, in the process of performing the re-oxidation process by use of the plasma processing apparatus of the RLSA type, if the chamber is contaminated with tungsten, oxidation of poly-silicon is inhibited and the thickness of an oxide film thus formed is decreased, although the mechanism for this is still unexplained. Where a plasma processing apparatus of the RLSA type is applied to a re-oxidation process, it is possible to provide many merits. However, if the oxide film thickness is decreased, semiconductor products suffer an increase in leakage current and so forth, which make it difficult to attain the original purpose of the re-oxidation.
[Patent Document 1]    Jpn. Pat. Appln. KOKAI Publication No. 11-293470 (Claims)